DESCRIPTION (from applicant's abstract): This is a revised application by Dr. David Camerini from the University of Virginia to study the role of CCR5-tropic HIV-1 isolates in HIV pathogenesis. The applicant will use the SCID-hu mouse model bearing human thymus/liver grafts to study the pathogenic effects of these isolates. Previously characterized R5 HIV-1 isolates from patients during pre-AIDS and AIDS stages of HIV-1 infection will be evaluated for the molecular determinants and mechanisms of pathogenesis in SCID-hu model. The R5-pre AIDS HIV-I isolates are not cytopathic to thymocytes whereas R5-AIDs isolates cause depletion of thymocytes in the SCID-hu model. The Specific aim 1 will be to map the regions of R5-AIDS which confer pathogenicity by developing recombinant infectious viruses. Initially, the focus will be on the envelope gene since it harbors molecular determinants for cell tropism and cytopathicity. The hypothesis to be tested is that changes in the env gene account for the increase in pathogenic potential of these isolates. The Specific aim 2 will test the hypothesis that these genetic changes could lead to increased affinity to CCR5 and/or broadened co-receptor usage by the R5-AIDs isolates. The recombinant viruses will be tested for the receptor usage. The Specific aim 3 will test the hypothesis that HIV-1 mediated signaling through CCR5 will allow infection of resting memory T cells and some thymocytes.